Thermionic cathode gas-filled discharge tube



Oct. 27, 1936. G. HOLST ET AL THERMIONIC CATHODE GAS-FILLED DISCHARGETUBE Filed Sept. 28, 1954 Invenfom:

GJTTOZSt,

E. Goaterkwda and JG. 501M um), 4 fl/ Patented Oct. 27, 1936 Gilles"Holst, Ekko JTHERMIONIC CATHODE DISCHARGE TUB PATENT OFFICE GAS-FILLEDOosterhuis, and Johannes Gijsbertus Wilhelm Mulder, Eindhoven,Netherlands, assignors to N. enfabricken, Eindhoven,

V. Philips Gloeilamp- Netherlands Application September 28, 1934, SerialNo. 746,000

In Germany October 27, 1933 2, Claims.

The present invention relates to electrical discharge tubes of the typehaving thermionic cathodes and a gas-filling and more particularly to acathode construction for such discharge tubes, which enables the tubestohandle large discharge currents.

' The term gas-filling as used herein also includes fillings of vapor orof mixtures of vapor and gas.

'The general purpose of using gas-fillings in discharge tubes of thistype, is to enable the tube to handle large currents without largelosses, whereby, due toionizationof the gas-filling the building up ofspace charge is prevented and the current conductivity of the tube isincreased by the participation'of the gas ions in the currentconduction.

, Such discharge tubes; in which the discharge has usually the characterof an arc, may be used, for instance, as rectifiers which are capable ofrectifying'large currents with small voltage loss, thus with highefiiciency, or may be used for various other purposes, for example asrelaytubes, was discharge lamps.

' However, when such tubes are required to handle very'large'currents,difliculties are encounteredfiFor instance, a cathodewhich is to provideanemission current of about 1,000amperes, requires a very large emittingsurface. If such a cathode is to be directly heated by the passage ofa-heating'current supplied from an external source-which in generaloffers the simplest solution -ithas to be given the shape of a singlefilament of a very great length. Consequently, a high heating voltagehas to be used, which brings about the known'difliculties of properinsulation, the danger of discharge between different portions of thefilament, the difliculty of positioning the lengthy filament, etc; Onthe other hand, if a lower heating voltage is used, cathodes of suchlarge cross-sections are required asto introduce serious constructionaldifiiculties.

A cathode construction according tothe inventionobviatesthe abovedifiiculties, and consists broadly of a composite cathode which consistsof a plurality of parallel connected individual cathode members. r

Such a general construction is well known for high vacuum tubes,- forinstance radio tubes, in

' which cathodes having a plurality of parallel extending and parallelconnected members have beenwusedi. these: members being usuallysurrounded by a common anode. However, the use of. similar.constructions.. ingas-filled .1 discharge tubes was doomed to failurebecause in such an arrangement it was impossible to achieve a uniformdistribution of the current among the individual cathode members. encesin-the emittingconditions caused one'or another of the members toproduce a slightly greater amount of emission current, whichuin turnincreased the temperature of this higher emitting member above those ofthe othersand thus further increased itsemission; the recurring cycle ofinfluences being continued, until the emitting;member was seriouslydamaged or even destroyed. a

The present invention is based onthe realization, that it ispossible toso design a cathode member that an accidental increase in itscurrent-emission does not cause any, or'at least no important, increaseinzlits temperature, and that it is possible to avoid'a localconcentration of the discharge arc. This will be more fully explainedhereinafter. j

The heating of any given. portionof a cathode filament, having alengthd:n is. influenced: by four diiferent causes. These are:

1. The heating current It passing through the filament-and supplied fromoutside sources. Assuming a resistance*dR for therfilamenti por* tion dxthe heatresulting from this source is If dR.

2. The heating up-of the cathodes-because of the. discharge arc. Theheat liberated at -,the cathode from this source is proportionalktd afunction of the arc voltage Eb(p) at this pointand to the current dIememitted by this element. The heating upof the cathode element dzrfromthis source canbe thusexpressed as fEmmdIem.

-3. .;'I '-he cooling-ofi of the cathode-element dzc because:oftheenergy consumed in producing the; emission as. This-is-alsoproportional to the current emittedby the element dx and can bedesignated as dIem.

4. The heating of the element da:'is dependentrupon the sum of the-various emission currents dIeio These currents pass 'theelement dr andare emitted by more distantportions -of-the cathode. The resultingheating of 5 the cathode element can be -expressed as v Where e -a:representsthedistance of the'farthest cathode element. from the element.under consideration.. x

Designating. the total heating of the t element Unavoidable differ- "dxby dW'r, the following equation is obtained: dW =1, dR+iE ,,d-I,,,,-d1,,,,+ f dI dR dW IfidR+f m m( (12) #9 As the externally supplied heat,i. e. IJZCZR remains constant, and the influence of the heat liberatedat the arc, namely fEbQa) is comparatively small and does not greatlyvary in practice, it immediately appears that the members in which andin which dlem dR appear, play the most important part in the heating up.

As the work function 15 of a cathode of a given composition is fixed, acompensation of the above-mentioned positive and negative members can beeffected by selecting dR small, 1. e., by giving the cathode element asmall resistance.

Evidently it is not necessary to have the member containing altogetherpredominate, so as to efiect a cooling off of the cathode elementwhenever the emission current increases, as there is no objection to thetemperature of a cathode element increasing with increasing emissioncurrents, as long as such increased temperature remains below a value atwhich damaging (possibly the melting) of this element would take place.

In a preferred embodiment of the invention, however, we keep theresistance of the individual parallel connected cathode memberssufficiently low as to effect a noticeable cooling-off of these memberswhenever the emission currents exceed their normal, permissible values.

This phenomenon of the cathode portions cooling off at excessive currentvalues, will be referred to as dimming.

To obtain a low cathode resistance for a given emitting surface area,the parallel-connected cathode elements are made short and thick. Thesedimensions can be conveniently designated by the value of the heatingvoltage applied to the elements. In practice we prefer to so form theparallel-connected cathode elements that the heating voltage across samedoes not exceed 1 volt.

When using a plurality of parallel-connected cathode elements care hasto be taken that the same heating voltage be applied to each cathodeelement.

In a preferred embodiment of the invention this is obtained by formingthe composite cathode of two parallel discs or rings between whichindividual, parallel-connected cathode elements are provided.

Each ring or disc is provided with a current lead which is so connectedto same that the voltage drop through the disc or ring, measured fromthe connection points of the current leads to any one of the connectionpoints of the cathode elements, has substantially the same value. Thiscan also be promoted by giving the discs and rings ample thickness.

Our invention will be illustrated in connection with some specificembodiments, which will be described in connection with the drawingforming part of this specification, in which:

Figure 1 is a schematic view of a cathode structure according to theinvention.

Fig. 2 is a discharge tube comprising a cathode structure in accordancewith the invention.

Fig. 3 is an enlarged perspective View of the cathode structure of thetube shown in Fig. 2.

Referring to Figure 1, there are shown four coiled wire cathode membersI, 2, 3, and 4, which are disposed parallel to each other, and which arealso parallel-connected between leads 5 and 6, the leads being providedwith preferably integral bent portions 5 and 6, which are secured,preferably by welding, to the respective ends of the filamentary cathodemembers. It should be noted that the leads 5 and 6 approach the group ofcathode members from opposite sides, 1. e., the lead 5 first connectswith the member 4 and then successively with members 3, 2 and l,whereasthe lead 6 first connects with cathode member l and then successivelywith cathode members 2, 3 and 4. As a result of such arrangement thecombined voltage drop in the two leads 5 and 6 is the same for eachcathode member and consequently the same heating voltage is appliedacross each of the cathode members. As it also appears from the drawing,the individual cathode members consist of a helical filament having onlya few turns.

The leads 5 and 6, as well as the cathode filament members I to 4,consist, as a rule, of a refractory metal, for instance tungsten, whichis preferably provided with a nickel coating. Furthermore, the filamentmembers may be surrounded by a closely wound nickel wire, as describedin U. S. patent to Dobben, et al. No. 1,718,123.

The cathode members are provided with an emissive coating, usually anoxide coating of high electron emissivity, for instance, with a bariumoxide coating. Instead of making the filament of tungsten, othersuitable metals or alloys, for instance nickel or nickel alloys, may beused.

The discharge tube shown in Fig. 2 is a rectifier tube comprising avitreous envelope 8. The envelope 8 is provided at one end with areentrant neck portion 20 in the bottom of which is sealed a metal ringII, preferably of chrome iron, the ring H carrying the anode 9.

The active portion of the anode 9 is formed of a hood-shaped graphitemember, into the central hollow portion of which extends a metal tubeIn, which tube is secured at its other and open end within the apertureof ring ll, for instance by being welded thereto. Through the metal tubelo cooling fluid may pass to the anode. The tube I0 is surrounded by atube l2 of insulating material, for instance of steatite, to prevent thetube l8 from participating in the discharge.

The composite cathode structure 1 consists of a plurality ofparallel-arranged and parallelconnected filamentary cathode members l3.In the construction shown, twelve of these members are arranged in equalspacing about a circle as indicated in Fig. 3 in dot and dash lines. Thecathode members 13 consist of comparatively short and heavy wire whichis coiled into a helix of a few turns and are of the general type abovedescribed. The cathode members I3 are connected at their upper end to adisc l4 and at their lower end to a ring l5, the disc I4 and ring l5being of considerable thickness and extending parallel to each other andperpendicularly to the cathode members I3. The disc I4 is provided witha heavy current lead 16, which passes centrally through the ring l5 andis connected at the center of the disc M.

The current lead NS for ring l5 forks into two branches H, which areconnected to two diametrically opposite points of ring l5.

Such an arrangement makes it possible to sufficiently insulate thecurrent leads from each other. Furthermore, because of the thicknessesof the disc I 4 and the ring l5, and the arrangement of the connectingpoints of the leads and the cathode members, the voltage drop betweenthe connecting points of the current leads and the connecting points ofthe individual cathode members is substantially the same. Thus an equalheating voltage across each cathode member is secured.

The current leads I 6 and l 8 are supported from the other end of thetube, which forms a neck 2| in the bottom of which are sealed two metaldiscs l9 and 20-preferably of chrome iron--to which the leads l6 and I8are respectively secured, preferably by welding.

As a gas filling of the discharge tube shown in Fig. 2, mercury vapormay be conveniently used, whereby an excess of liquid mercury is alsoprovided in the tube. Preferably the temperature at the mercury shouldbe such as to give approximately 10 millimeters of mercury vaporpressure.

In one specific type of tube made in accordance with the invention, eachof the cathode members is dimensioned for a heating voltage of 0.8 voltsand a heating current of about 10 amperes. With such cathodeconstruction a very high degree of uniformity in the burning of theparallel-connected cathode members is obtained. This uniformity can beachieved in tubes for very large currents, for example, 800 amperes andmore, built according to our invention. Such cathodes preferably exhibitthe above referred dimming phenomenon, i. e. when the emission currentin a cathode member exceeds the permissible value a lowering of thetemperature of this member takes place.

While we have described our invention in connection with a specificapplication and specific embodiments, we do not wish to be limitedthereto. For instance, the discharge tubes of the invention can be usedfor other purposes, and various deviations of the specific construction,of the gas filling used, etc., may take place without departing from thespirit of our invention.

We wish, therefore, the appended claims to be construed as broadly aspermissible in view of the prior art.

What we claim is:

1. A cathode structure for a thermioniccathode, gas-filled dischargetube comprising a group of parallel-disposed and parallel-connectedcoiled wire cathode elements, a conductive member connected to one endof each of said cathode elements and a conductive member connected tothe opposite ends of said cathode elements, and a current lead connectedto each of said conductive members, said leads approaching said groupfrom opposite sides thereof, the combined voltage drops in saidconductive members between connecting points of said leads and theconnecting points of said cathode elements being the same for each ofsaid cathode members.

2. A composite cathode structure for thermionic-cathode, gas-filleddischarge tubes, comprising a disc member and a ring member disposedparallel to each other, a plurality of coiled wire cathode elementsmounted in parallel to each other and equidistantly spaced along theperiphery of said members, said elements being connected in parallelbetween said members, a current lead centrally connected to said disc,and a current lead for said ring having two branches, said. branchesbeing connected to diametrically opposite points of said ring.

GILLES HOLST.

EKKO OOSTERI-IUIS.

JOHANNES GIJSIBERTUS WILHELM MULDER.

